Low‐Power Negative‐Differential‐Resistance Device for Sensing the Selective Protein via Supporter Molecule Engineering

Abstract Van der Waals (vdW) heterostructures composed of atomically thin two‐dimensional (2D) materials have more potential than conventional metal‐oxide semiconductors because of their tunable bandgaps, and sensitivities. The remarkable features of these amazing vdW heterostructures are leading to multi‐functional logic devices, atomically thin photodetectors, and negative differential resistance (NDR) Esaki diodes. Here, an atomically thin vdW stacking composed of p‐type black arsenic (b‐As) and n‐type tin disulfide (n‐SnS2) to build a type‐III (broken gap) heterojunction is introduced, leading to a negative differential resistance device. Charge transport through the NDR device is investigated under electrostatic gating to achieve a high peak‐to‐valley current ratio (PVCR), which improved from 2.8 to 4.6 when the temperature is lowered from 300 to 100 K. At various applied‐biasing voltages, all conceivable tunneling mechanisms that regulate charge transport are elucidated. Furthermore, the real‐time response of the NDR device is investigated at various streptavidin concentrations down to 1 pm, operating at a low biasing voltage. Such applications of NDR devices may lead to the development of cutting‐edge electrical devices operating at low power that may be employed as biosensors to detect a variety of target DNA (e.g., ct‐DNA) and protein (e.g., the spike protein associated with COVID‐19).


Electrical characterization of b-As and SnS 2
The electrical characteristics of each material were analyzed to investigate their intrinsic nature. At a fixed V ds of 0.5 V, the gate-dependent transfer curves indicated the n-type and p-type natures of SnS 2 and b-As, respectively, as shown in Figures S1a and S1b. The charge carrier densities of the electrons in SnS 2 and holes in b-As were estimated using - [1][2] . The electron carrier density was calculated to be approximately 2.1×10 11 /cm 2 at V g = 0 V, with a high on/off current ratio of ~10 5 , while the hole carrier density in b-As was estimated to be approximately 1.12×10 11 /cm 2 at V g = 0 V. The output curves for SnS 2 and b-As were investigated, and the non-linear I ds -V ds curves at zero gate voltage show the Schottky barrier contribution of the Cr/Au electrodes for both SnS 2 and b-As (Figures S1c and S1d).
3 Figure S1. (a) The gate-dependent transfer curve extracted from the n-type SnS 2 flake and (b) ptype b-As, at a fixed bias voltage of V ds = 0.5 V. (c) Output curve of n-type SnS 2 and (d) p-type b-As flake, at zero gate voltage and room temperature.
Furthermore, charge transport through the vdW heterostructure was studied at low temperatures to verify the tunneling mechanism between b-As/SnS 2 . If the plot of ln (I/V 2 ) vs.
(1/V) shows an exponential increase at higher V ds then it represents direct tunneling, whereas the

Material characterization
Solid unconjugated streptavidin was purchased from Thermo Fisher Scientific, and a 100 µM solution was prepared, followed by serial dilutions using deionized water (DIW). The concentration was confirmed by measuring the absorbance of the solution (at 280 nm) using a spectrophotometer ( Figure S4a) and calculating the concentration using the Beer-Lambert law.
The synthesized PLB construct was characterized by UV spectroscopy, as shown in Figure S4b.

Raman spectra analysis of channel material
Raman spectra were recorded at room temperature using a 532 nm laser via a 50x (100x) objective lens. The spectra were recorded for a device containing pristine SnS 2 , after functionalization with our pyrene-based receptor, and finally after capturing the target biomolecule (streptavidin). The spectra of bare SnS 2 show clear and sharp peaks at ~315.4 cm -1 (A 1g ) and ~200 cm -1 (E g ), indicating the pristine nature of the material [3][4] . These peaks represent two Raman-active phonon modes of 2H-phase SnS 2 [5] . The presence of a single peak in the 190-225 cm -1 range represents pure single-crystal SnS 2 in its ground state with 2H polytype. By applying a Gaussian fit, the full width at half maximum (FWHM) of the main resonance peak (A 1g ) was estimated to be 9.85, representing the crystalline nature of the material. Furthermore, the sharp second resonance peak at 200 cm -1 represents a small number of layers of SnS 2 , whereas the broad peak at this position is attributed to a large number of layers. Because of the pristine nature of the SnS 2 material, the intensity ratio A 1g /E g was very high (101.75), as shown in Figure S5a. Moreover, the properties of the peaks were plotted for three different positions and are consistent in Figure S5b. However, after functionalizing pure SnS 2 with our pyrenebased supporter construct, the peak properties changed significantly. The crystallinity represented by A 1g is absent, which results in an increase in its FWHM from 9.85 to 10.40.
Moreover, the second resonance peak (E g ) appears predominantly, representing the presence of a multilayer due to the attachment of our supporter construct onto SnS 2 , eventually resulting in a sharp decrease in the peak intensity ratio A1 g /E g from 101 to 36.80. Furthermore, upon applying the solution containing our target protein, the functionalized channel successfully captured it, owing to the inherent binding affinity between biotin and streptavidin. Because of the streptavidin capture, the FWHM of the A 1g peak increased to 12.70, and the peak intensity ratio 8 (A 1g /E g ) further decreased to 19.07. The results were consistent for multiple measurements, as shown in Figure S5b.
Similarly, the Raman spectra of the device channel containing mechanically exfoliated b-As were recorded at room temperature using a 532 nm laser via a 50x (100x) objective lens. Pure b-As consists of 12 lattice vibrational modes at the G-point, of which six are Raman active with irreducible representation. However, our results revealed two distinct peaks at 225 (B 2g ) and 257 (A 2g ) cm -1 , consistent with a previously published report [6] . Here, B 2g represents the out-of-plane vibrations of the material and A 2g represents the in-plane vibrations. After functionalizing b-As with our PLB construct, the peak properties changed significantly. The main resonance peak (A 2g ) blue-shifted from 256.57 to 253.51 cm -1 , indicating a clear increase in the layer thickness upon functionalization. It can also be observed that the intensity ratio A 2g /B 2g decreases from 1.68 to 1.66 upon functionalization. The shift in the peak position after the attachment of the pyrene-based support is attributed to the sharing of electrons. Pyrene has a higher electron density than b-As, and charge is transferred to b-As upon functionalization. The weakening of the peak intensity and its shifting can be more clearly observed after streptavidin capture. The intensity of A 2g /B 2g decreased to 1.54, while a clear blue shift in the A 2g peak can be observed in Figure S5c. This large change is due to the relatively large size of the streptavidin protein (5.2 nm), as compared to the pyrene-based supporter construct (~1.5 nm). The attachment of streptavidin results in the effect of bulk b-As, resulting in broadening of peaks, and this observation is in agreement with previously published literature [7] . The properties of the peaks were plotted at three different positions, as shown in Figure S5d.  Table S1. The details of statistical analysis for Figure 2d. Here, the gate voltage was our variable against which the peak-to-valley current ratios (PVCRs) were measured. To avoid errors, each set of measurements was repeated for three different devices. Table S2. The details of statistical analysis for Figure 3c. Here, the temperature was our variable, and its effect on the peak-to-valley current ratio (PVCR) was measured. Each set of measurements was repeated for three different devices to minimize error. Table S3. The details of statistical analysis for Figure 5f. Here, the response percentage of our sensor was our variable, which was measured during the device performance testing against various targets (streptavidin) and un-wanted protein (BSA), and each set of measurements was repeated for three different devices.